A switching regulator is conventionally used for a power circuit of portable equipment which reproduces a compact disc (CD), mini disc (MD) or the like to output a power output voltage obtained by converting the voltage of a battery which is a supply voltage source on the primary side to a set voltage to a load on the secondary side. For example, JP A2001-69749 proposes a switching regulator shown in FIG. 11.
The conventional switching regulator shown in FIG. 11 uses the construction of a voltage step up DC/DC converter. That is, the present switching regulator steps up a power input voltage (battery voltage) VIN from a battery which is a supply voltage source BT on the primary side to a set voltage through a switching operation of a bipolar transistor TrA which is a coil drive transistor and field effect transistor (power MOSFET) TrB and outputs it to the load on the secondary side.
In FIG. 11, the detected voltage of a power output voltage VOUT on the secondary side which is detected at a division ratio of resistors R2, R3 is input to an FB terminal of a driver IC 101 for a DC/DC converter. Also, the power output voltage VOUT is input to a VDD terminal as the supply voltage for the driver IC 101. Furthermore, a pulse signal which controls the switching operation of the coil drive transistor is output from an OUT terminal. FIG. 12 shows the pulse signal output from the OUT terminal.
The driver IC 101 controls an “H” level period (Ton) and “L” level period (Toff) of the pulse signal output from the OUT terminal based on the difference between the detected voltage input to the FB terminal and an internal reference voltage and performs negative feedback operation so that the voltage value obtained by dividing the power output voltage VOUT using resistors R2, R3 converges toward the internal reference voltage. Furthermore, the driver IC 101 sets a pulse peak value Vm of the pulse signal output from the OUT terminal to substantially the same potential as the VDD terminal voltage and outputs it.
A PWM pulse control section 102 converts the pulse signal output from the OUT terminal of the driver IC 101 to a current through a resistor R1, outputs it to the base terminal of the bipolar transistor TrA and directly outputs the pulse signal to the gate terminal of the field effect transistor TrB. The bipolar transistor TrA and field effect transistor TrB are ON for a period during which the pulse signal is at an “H” level (Ton) and OFF for a period during which the pulse signal is at an “L” level (Toff).
Hereinafter, the operation of this conventional switching regulator will be explained.
First, upon start-up of the supply voltage source BT, if a forward diode voltage of a Schottky barrier diode D1 is assumed to be 0.2 (V), a voltage of:VIN−0.2is applied to the VDD terminal of the driver IC 101, where VIN is a power input voltage (battery voltage).
Therefore, if it is assumed that the base-emitter voltage of the bipolar transistor TrA is VBE and the resistance value of resistor R1 is R1, the base current Ib input to the base terminal of the bipolar transistor TrA upon start-up is,Ib≈(VIN−0.2−VBE)/R1and the bipolar transistor TrA starts the switching operation.
Furthermore, if a threshold voltage of the field effect transistor TrB is assumed to be Vth, when the power input voltage (battery voltage) VIN upon start-up has a relationship:VIN−0.2<Vth the relationship between the pulse peak value Vm of the pulse signal output from the OUT terminal of the driver IC 101 and the threshold voltage Vth is also similar, and therefore the field effect transistor TrB whose threshold voltage Vth is large cannot start the switching operation immediately after start-up.
After start-up, when the power output voltage VOUT is stepped up by the switching operation of the bipolar transistor TrA so as to have a relationship:VOUT≧Vththe relationship between the pulse peak value Vm of the pulse signal output from the OUT terminal of the driver IC 101 and the threshold voltage is also similar, and therefore the field effect transistor TrB starts the switching operation.
As shown above, the conventional switching regulator provided with the bipolar transistor and field effect transistor connected in parallel as coil drive transistors steps up the power output voltage VOUT on the secondary side to a predetermined voltage (threshold voltage of the field effect transistor) through the switching operation of the bipolar transistor upon start-up first, and then steps up the power output voltage VOUT on the secondary side to a set voltage mainly through the switching operation of the field effect transistor.
Furthermore, though the internal construction of the driver IC 101 for the DC/DC converter is not known in FIG. 11, the construction of a general voltage step up DC/DC converter is as shown in FIG. 4. With the general voltage step up DC/DC converter, the power output voltage VOUT is controlled so that the voltage value obtained by dividing the power output voltage VOUT using resistors 41, 42 converges to an internal reference voltage Va of the reference voltage source 44.
However, according to the construction of the conventional power apparatus, even when the apparatus is used for electrical equipment having a standby mode and normal operating mode, the power input voltage is converted to a set voltage by the same circuit in the standby mode and normal operating mode. That is, the same set voltage is supplied to a load with the same current supply performance. For this reason, even when a device (e.g., microcomputer) whose current consumption in a standby mode is smaller than in a normal operating mode is used as a load on the secondary side, the construction of the conventional switching regulator cannot reduce current consumption of the switching regulator itself in the standby mode, unable to reduce current consumption (outflow current) from the supply voltage source (battery) in the standby mode.
Furthermore, when a device requiring an accurate and constant voltage (e.g., microcomputer) as the power supply input in the normal operating mode is used as the load on the secondary side and a battery whose voltage drops with time is used as the supply voltage source on the primary side, the following problem occurs with the conventional switching regulator made up of only a voltage step up DC/DC converter. That is, for the conventional switching regulator, when the battery voltage is higher than the voltage obtained by adding the forward diode voltage of the rectification diode D1 to the set voltage, the power output voltage to be supplied to the load is raised to a voltage obtained by subtracting the forward diode voltage of the diode D1 from the battery voltage. When the battery voltage decreases with time, the power output voltage decreases accordingly and it is not possible to supply a constant voltage to the device in the normal operating mode.